Trolling motors that utilize electronic variable speed control require power devices which have some level of resistance and must dissipate heat. This heat must be thermally conducted away from the internal devices to the external environment. Depending on the trolling motor size, large amounts of metal can be required for heat sinking, which is usually aluminum because of its high thermal conductivity. However, aluminum adds significant cost, especially if the controller head housing must be metal in order to assist with the heat sinking. The area and volume of heat sinking material required is also a significant factor in determining the head housing size and styling limitations.
An important limitation for heat sinks in a trolling motor head is the lack of ventilation or air flow across a heat sink that is mounted to the internal power devices because trolling motors need a good level of sealing to protect water from entering the head during use on water ways, during rain, and when washing the unit after use.
Heat sinks can be classified in terms of manufacturing methods and their final form shapes. The most common types of air-cooled heat sinks include:
Stampings—Copper or aluminum sheet metals are stamped into desired shapes. They are used in traditional air cooling of electronic components and offer a low cost solution to low density thermal problems. The problem for trolling motors is the lack of space and/or air flow across the stamped shape.
Extrusions—The most common heat sink for larger power devices because extrusions allow the formation of elaborate two-dimensional shapes capable of dissipating large heat loads. They may be cut, machined, and options added. Cross-cutting will produce omni-directional, rectangular pin fin heat sinks, and incorporating serrated fins improves the performance by approximately 10%-20%. Extrusion limits, such as the fin height-to-gap ratio and fin thickness, usually dictate the flexibility in design options. Typical fin height-to-gap aspect ratio is of up to 6:1 and a minimum fin thickness of about 1.3 mm are attainable with standard extrusion processes. Air flow across the fins is not possible in a sealed trolling motor.
Bonded/Fabricated Fins—Most air cooled heat sinks are convection limited, and the overall thermal performance of an air cooled heat sink can often be improved significantly if more surface area can be exposed to the air stream. High performance heat sinks utilize thermally conductive aluminum-filled epoxy to bond planar fins onto a grooved extrusion base plate. This process allows for a much greater fin height-to-gap aspect ratio of 20 to 40, greatly increasing the cooling capacity without increasing volume requirements. Large fin area requires the trolling motor to have an equally large head design.
Folded Fins—Corrugated sheet metal in either aluminum or copper increases surface area and the volumetric performance. The heat sink is then attached to either a base plate or directly to the heating surface via epoxy or brazing. It is not suitable for high profile heat sinks, but it allows high performance heat sinks to be fabricated for specific applications. Large fin area requires the trolling motor to have an equally large head design.
Previous designs for transferring the heat from power electronics of a trolling motor controller in the head have utilized a flat plate embedded in the housing or a completely metal housing, with the power electronics attached to the heat sink.